1. Field of the Invention
The present invention relates to a neck-in control device of a flat die. More specifically, it relates to a neck-in control device of a flat die capable of minimizing neck-in generated in a molten resin extruded from a flat die (or T-die) during film extrusion molding in, for example, extrusion laminate molding.
2. Description of the Related Art
Film extrusion molding methods in which molten resins are extruded in the form of a film from flat dies, such as so-called straight type and coat hanger type dies, are widely known in the art and are utilized for the production of films per se and for the extrusion lamination molding used to laminate base materials with the extruded films.
However, when molten resins flow from dies, the so-called "neck-in" phenomenon occurs. Neck-in is a phenomenon in which the width of molten resin films extruded from the dies becomes narrow and the edge portions of the films become thick, as shown in FIG. 2.
The neck-in phenomenon is caused by the combined force of a surface tension of the molten resin around the die portion, the melt elastic effect, and the tensile stress of the molten resin towards the take-off direction. This phenomenon is always generated, although the extent thereof depends upon, for example, the type and molecular weight of the thermoplastic resins utilized. Furthermore, in addition to the neck-in phenomenon, so-called wavy edges (i.e., waving or undulation occurs in both edge portions of the extruded film) are generated as the molten resin film is extruded.
Especially, thermoplastic resins having a low melt tension such as polyethylene terephthalate, polypropylene, poly 4-methyl-1-pentene, and polyamide exhibit a large neck-in phenomenon and easily tend to easily incur wavy edges. When the wavy edge phenomenon occurs, both edges of the film are formed with undulations and, therefore, coating the film on a base material or substrate becomes impossible. Thus, the edge heads generated by the neck-in phenomenon during the molding must be trimmed by a cutter at a subsequent fabrication step.
Accordingly, when the neck-in is large, the width of the resultant film becomes narrow and the thickness of the both edges becomes thick. As a result, the trimming width becomes large and, therefore, the available film width is remarkably smaller than the width of the die and much of the extruded film is wasted. Furthermore, a remarkably large sized die must be used to obtain a film having a required width. Consequently, there is a strong need or desire in the art that neck-in should be minimized. Especially, in extruding lamination molding, since the extruded film is successively coated onto a base material such as paper, the extrusion of a film having a width identical to that of the base material, without trimming, is greatly desired.
Furthermore, the so-called draw-down properties (i.e., high speed processability) are generally required in the extruding lamination molding. However, the physical properties of resins (e.g., the above-mentioned thermoplastic resins having a low melt tension) and the processing conditions suitable for improving the draw-down properties increase the occurrence of the neck-in phenomenon.
Accordingly, it is strongly desired in the art to reduce neck-in, while improving the draw-down properties.
Various attempts have been made to reduce neck-in. For example, a method in which the distance between the resin extruding outlet of a die and a chill roll (i.e., a so-called "air gap") is made short, is often used, as it is the most simple method. However, since in the extruding lamination molding it is necessary to extrude a molten resin to or around the contact line of a chill roll and a nip roll, there is necessarily a limit to the shortening of the air gap distance, in view of the size of the die head, and therefore, neck-in cannot be fully eliminated by this method.
Accordingly, the extent of the neck-in is generally adjusted (but not prevented), at present, by moving the deckel plate and adjustment rod backward and forward.